The present invention relates to devices for detecting atomic or nuclear particles and, more particularly, to semiconductor particle detectors.
During the investigation of nuclear reactions brought about in the laboratory, it is necessary to detect and to identify the products of nuclear reactions, which may be ions of atoms or nuclear fission fragments with higher or less high atomic numbers, or alpha particles. Similar requirements generally arise during analysis of radioactive phenomena with the emission of high-energy particles, as in cosmic-ray analysis.
In order to identify a particle, it is necessary to measure various quantities such as electric charge, kinetic energy, vectorial momentum, and atomic number. A known method of measuring charge and energy provides for the use of two superimposed semiconductor detectors, of which one is relatively thick (300-400xcexcm) and one is relatively thin (from a few xcexcm to a few tens of xcexcm). An incident particle interacts first with the thin detector, losing only some of its energy (xcex94E) and then with the thick detector to which it yields all of its residual energy (Exe2x88x92xcex94E), where E represents the energy of the particle before impact with the thin detector.
The manufacture of xcex94E detectors is quite difficult, particularly when detectors with a relatively extensive active area are required as is the case for position detectors, that is, detectors which can also provide information on the spatial distribution of the incident particles. In these cases, it is necessary to form plates of semiconductor material with an area of as much as a few hundred square centimeters and a thickness of only about ten xcexcm, which would therefore be extremely fragile and difficult if not impossible to handle.
Various techniques have been proposed for the manufacture of detectors of this type. One of these (described in an article by G. Thungstrom et al., in Nuclear Instr. and Meth. in Ph. Res., A 391 (1997) 315-328) provides for joining of two superimposed silicon wafers with the use of a metal silicide as a joining and interface layer, reduction of the thickness of one of the wafers to achieve the desired thickness for the xcex94E detector and passivation of the free surfaces of the wafers thus joined, opening of windows for the active areas of the detector and for the connection to the metal silicide interface layer, and implantation and diffusion of doping impurities to form junctions.
The resulting structure is shown schematically and in section in FIG. 1. It includes a first, n-type monocrystalline silicon wafer 10 with a low concentration of impurities (nxe2x88x92xe2x88x92), for example, phosphorus, having a surface layer 11 with a higher concentration of n-type impurities (for example, arsenic) and by a second silicon wafer 9, which is also n-type with a low concentration of impurities (phosphorus), and which is joined to the first wafer by a layer 12 of cobalt silicide. Two planar p-type regions 13 and 14 are formed on the free surfaces of the two wafers and metal contacts 15 and 16 which form two electrodes of the detector are formed thereon. A third electrode is formed by a contact 17 on the cobalt silicide layer 12. From an electrical point of view, the structure is equivalent to a pair of diodes having a common cathode (the layer 12). The upper, thin diode forms the xcex94E detector and the lower, thick diode forms the xcex94-E detector.
In operation, two voltages, xe2x88x92V1 and xe2x88x92V2 relative to a common terminal represented by the ground symbol in the drawing, are applied between the upper and lower electrodes 15 and 16, respectively, and the intermediate electrode 17, the voltages being of a sign such as to bias the two diodes in reverse. Two depletion regions are thus formed between the two p regions and the two n layers adjacent thereto, which form the active portions of the two detectors. A particle to be detected, represented by an arrow with a broken line, strikes the front surface of the detector and, as it passes through the two depletion regions, brings about the formation of electron-hole pairs which move towards the electrodes, giving rise to pulsed currents. These currents are collected and amplified by suitable circuits, generally indicated 18 and 19, and are then measured and displayed by a suitable electronic device, indicated 7, to give an indication of the quantities xcex94E and Exe2x88x92xcex94E and hence of the masses of the incident particles.
With the known detector described above, the portion which detects xcex94E can be made very thin while avoiding the problems of fragility indicated above because it is processed when it is fixed to the much thicker portion which detects Exe2x88x92xcex94E. However, this detector is not suitable for mass production because it requires processing steps which are not provided for in normal processes for the manufacture of monolithic integrated circuits. Moreover, it cannot function as a position detector.
A structure which can be produced by standard techniques and which can be used as a position detector is described in patent publication EP-A-0730304 and is shown schematically in FIG. 2. As can be seen in the drawing, the detector is formed in a single monocrystalline silicon chip 20 which comprises three superimposed layers: two n-type layers, that is, an upper layer 22 and a lower layer 23, and one p-type layer 21 which is strongly doped and is therefore marked P+, and which is xe2x80x9cburiedxe2x80x9d, that is, interposed between the two n-type layers, and extends to a certain distance from the lateral surface of the chip. In order to contact the p+layer 21 electrically, a region 24, which is also a strongly doped p-type region is provided, extending from the upper surface of the chip to the p+ layer 21 and having, on the surface, a contact element in the form of a metal strip 27, for example, an aluminium strip. Seen in plan, the p+ contact region 24 and the metal strip 27 are typically in the form of square or rectangular frames.
The upper layer 22 and the lower layer 23 are contacted electrically by two n-type surface regions which are strongly doped and are therefore marked N+ and indicated 31 and 32, respectively, and by two metal layers 25 and 26, respectively. The latter, together with the metal strip 27, form the terminal electrodes of the detector and serve to connect the detector to biasing voltage sources and to circuit amplifiers, processors and indicators similar to those already described briefly with reference to FIG. 1.
The above-described structure is produced by the usual manufacturing processes for planar technology. This enables the detector to be mass produced, although with some difficulties and with results which are not always completely satisfactory. In particular, it is the formation of the buried layer which creates some problems. It can in fact be produced by high-energy implantation of boron ions directly at the desired depth in the substrate, or by surface doping of the substrate followed by the formation of the n layer 22 by epitaxial growth. In the first case, the high-energy implantation is a fairly critical and potentially harmful operation and, in the second case, when operating with the usual monocrystalline silicon substrates with  less than 111 greater than  crystallographic orientation and with boron as the dopant, phenomena of epitaxial shift (epi-shift), self-doping and disappearance of the alignment marks for identifying the limits of the buried geometrical arrangements occur. This leads to an enlargement of the buried layer which is difficult to control.
The structure of FIG. 2 can be modified, as shown in FIG. 3, to operate as a position detector. The modification includes the formation of elemental cathode electrodes 31a-31d distributed over the active surface of the xcex94E detector and of the connection of each elemental cathode electrode to the detection circuit by a respective metal contact 25a-25d. This considerably increases the complexity of the manufacturing process.
An object of the present invention is to provide a position detector which does not require a large number of connections.
Another object is to provide a method of manufacturing a xcex94Exe2x88x92E detector which provides solely for standard planar technology processing steps but which is not subject to collateral defects described above with reference to the manufacture of the structure of FIG. 2.
These and other objects are achieved by a chip of semiconductor material including a first layer with a first type of conductivity having a surface on the first major surface of the chip, a second layer with the first type of conductivity having a surface on the second major surface of the chip, and a third layer with the first type of conductivity having a resistivity lower than those of the first and second layers and disposed between the first layer and the second layer. A first region with a second type of conductivity, extends from the first surface into the first layer, and a second region with the second type of conductivity, extends from the second major surface into the second layer. First, second and third electrical connections are provided for connection with the first region, the second region, and the third layer, respectively. To provide a position detector which does not require a large number of connections, the second electrical connection includes two electrodes arranged a predetermined distance apart on the surface of the second region.